1. Field of the Invention
The invention relates generally to the field of drill bits used to drill earth formations. More specifically, the invention relates to methods for designing, and to designs, for drill bits having improved drilling performance.
2. Description of the Related Art
Roller cone drill bits used to drill wellbores through earth formations generally include a plurality of roller cones rotatably mounted to a bit body. The bit body is turned by a drilling apparatus (drilling rig) while axial force is applied to the bit to drill through the earth formations. The roller cones include a plurality of cutting elements disposed at selected locations thereon. The types, sizes and shapes of the cutting elements are generally selected to optimize drilling performance of the drill bit in the particular earth formations through which the formation is to be drilled.
The cutting elements may be formed from the same piece of metal as each of the roller cones, these being so-called “milled tooth” bits. Other types of cutting elements consist of various forms of “inserts” (separate bodies formed from selected materials) which can be affixed to the roller cones in a number of different ways.
Some types of cutting elements, both milled tooth and insert type, have cutting edges (“crests”) which are not symmetric with respect to an axis within the body of the cutting element. These are called non-axisymmetric cutting elements. Some types of roller cone drill bits have non-axisymmetric cutting elements oriented so that the crests are oriented in a selected direction. The purpose of such crest orientation is to improve the drilling performance of the roller cone bit.
One such method for improving drill bit performance by orienting cutting element crests along a particular direction is described in published patent application PCT/US99/19992 filed by S. Chen. The method disclosed in this application generally includes determining an expected trajectory of the cutting elements as they come into contact with the earth formation. The expected trajectory is determined by estimating a rotation ratio of the roller cones, this ratio being the cone rotation speed with respect to the bit rotation speed. The crests of the cutting elements are then oriented to be substantially perpendicular to, or along, the expected trajectory. Whether the crests are oriented perpendicular or along the expected trajectory depends on the type of earth formation being drilled.
Yet another method for orienting the crests of the cutting elements on a roller cone bit is described in U.S. Pat. No. 5,197,555 issued to Estes. As explained in the Estes '555 patent, the crests of the cutting elements are oriented within angle ranges of 30 to 60 degrees (or 300 to 330 degrees) from the axis of rotation of the cone.
It is desirable to provide a drill bit wherein non-axisymmetric cutting elements are oriented to optimize a rate at which the drill bit cuts through earth formations.